1. Field of the Invention
The present invention relates to semiconductor photodetector devices such as photodiodes and avalanche photodiodes.
2. Background Art
In a multiple-wavelength optical communications system, the optical receivers must have a function to selectively receive light having a desired wavelength.
FIG. 11 is a diagram showing the configuration of a conventional optical receiver. In the figure, two wavelengths of light, 1.3 μm and 1.55 μm, are incident on the optical receiver, but the avalanche photodiode 161 receives only the 1.3 μm wavelength light. Specifically, a wavelength filter 163 for reflecting 1.55 μm wavelength light 162 is provided in front of the avalanche photodiode 161, acting as a photodetector device, to selectively receive 1.3 μm wavelength light 164.
FIG. 12 is a cross-sectional view of a conventional avalanche photodiode (hereinafter referred to as a “conventional APD”) for optical communications. Referring to the figure, reference numeral 171 denotes an anode electrode; 172 denotes a p-type diffusion layer region; 173, a nonreflective film; 174, an undoped InP window layer; 175, an n-type InP electric field reduction layer; 176, an undoped InGaAsP graded layer; 177, an undoped InGaAs light absorption layer; 178, an n-type InP substrate; 179, a cathode electrode; 180, an anode electrode; 181, a multiplication region; and 182, a guard ring region.
The nonreflective film 173 and the InP window layer 174 also act as a surface protective film and a multiplication layer, respectively. It should be noted that the InP window layer 174 has a large bandgap and hence does not absorb the wavelengths used in typical optical communications, such as 1.3 μm and 1.55 μm, allowing these wavelengths to pass without change. The guard ring region 181 is provided to prevent edge multiplication and is a p-type region having a low carrier concentration.
Light entering the nonreflective film 173, as shown at the top of the FIG. 12, is passed through the InP window layer 174 and then absorbed by the InGaAs light absorption layer 177, generating electrons and holes. It should be noted that the avalanche photodiode (APD) is reverse-biased with a high voltage (approximately 25 V), which depletes the InGaAs light absorption layer 177, the InGaAsP graded layer 176, the n-type InP electric field reduction layer 175, and the multiplication region 181. Therefore, the generated electrons flow toward the n-type InP substrate 178 through the depleted layers. On the other hand, the holes flow toward the multiplication region 181 having a high electric field applied thereto. The holes that have reached the multiplication region 181 causes avalanche multiplication, generating a large number of new electrons and holes. As a result, the light signal that has entered the APD is drawn from it as a multiplied electric current signal. The magnitude of the obtained electric current signal is ten-odd times larger than when no multiplication occurs.
Further, there is a conventional semiconductor photodetector device which, upon reception of two different wavelengths of light, photoelectrically converts only the longer wavelength light and outputs the resultant signal (see, e.g., Japanese Patent Laid-Open No. 2000-77702). That is, this semiconductor photodetector device has sensitivity to only the longer wavelength light.
FIG. 13 is a cross-sectional view of this semiconductor photodetector device. Referring to the figure, reference numeral 191 denotes an n−-type InGaAs second absorption layer; 192, an n-type InP buffer layer; 193, an InGaAsP first absorption layer; 193a, a p-type InGaAsP region; 193b, an n−-type InGaAsP region; 194, an n-type InP substrate; 195, an antireflective film; 196, a p-type diffusion layer region; and 197, a nonreflective film.
The following description assumes that 1.3 μm wavelength light and 1.55 μm wavelength light are incident on the photodetector device shown in FIG. 13. In the photodetector device, the 1.55 μm wavelength light, whose wavelength is longer than the bandgap wavelength of the InGaAsP first absorption layer 193, reaches the n−-type InGaAsP second absorption layer 191 and then is drawn from the device as a photocurrent. On the other hand, the 1.3 μm wavelength light, whose wavelength is shorter than the bandgap wavelength of the InGaAsP first absorption layer 193, is absorbed by the InGaAsP first absorption layer 193. In this case, since no electric field is applied to the InGaAsP first absorption layer 193, the generated carriers recombine with each other. Therefore, this shorter wavelength light is not drawn from the device as a photocurrent.
Further, there is a conventional technique in which a reflective film for reflecting the shorter wavelength light is formed to receive only the longer wavelength light and convert it into a photocurrent (see, e.g., Japanese Patent Laid-Open No. 2002-33503). This photodetector device also has sensitivity to only the longer wavelength light.
Incidentally, recent multiple-wavelength optical communications systems require optical receivers having a very high wavelength selectivity ratio as much as 1000:1, or 30 dB, for 1.3 μm and 1.55 μm wavelengths. This means that these optical receivers must have high sensitivity to 1.3 μm wavelength light but substantially no sensitivity to 1.55 μm wavelength light.
However, to achieve such a high selectivity ratio, conventional APDs must be provided with a wavelength filter, as described below.
Referring to FIG. 12, the bandgap wavelength of the InGaAs light absorption layer 177 is 1.67 μm, and that of the InP window layer 174 is 0.92 μm. Therefore, this APD has high sensitivity to a wide range of wavelengths, from 0.92 μm to 1.67 μm, which means that the APD has approximately the same sensitivity to 1.3 μm and 1.55 μm wavelengths. As a result, the APD cannot receive the shorter wavelength 1.3 μm without receiving the longer wavelength 1.55 μm unless it is provided with a wavelength filter.
Further, as described above, although photodetector devices for selectively receiving the longer wavelength light have been available, there is no known photodetector device capable of selectively receiving the shorter wavelength light.